I think they cleared out a lot of 'dead wood' and thats why the number decreased.
If eBay cleared out the duplicate auctions,scams and overpriced rubbish from China their figure would drop a lot more than a million.The eBay user figures are pretty inaccurate,a pal of mine still has an active accout but died a couple of years back.At least he doesn't have to worry about all the rule changes.

This happens on occasion on eBid. They have a clear out of sellers who are not performing. Within the last week one of the biggest sellers has been removed for non performance. This will greatly enhance the site for genuine sellers whose listings were swamped by a bad seller with high negartive feedback.